(1) Field of the Invention
The present invention relates generally to a system and method for processing an optical information with a tracking error compensated.
The present invention more specifically relates to the optical information processing system and method which record and/or reproduce the optical information on and/or from record tracks on a card or tape type optical record medium using a particular acoustooptic deflector (AOD), with tracking error detected and compensated.
(2) Background of the Art
Various kinds of optical information processing systems have recently been proposed.
In one of such optical information processing systems, a light spot formed of, e.g., a converged laser light flux is used to scan, e.g., over a tape type optical record medium in a direction transverse to a direction toward which the tape type record medium runs so that the optical information is written or read on each record track.
A diameter of the light spot can be converged to, e.g., approximately one micrometer, in a case where the laser light having a waveform length of 780 nanometers is used.
Hence, if the multiple record tracks are formed with, for example, guard bands sandwiched between each record track and the optical information can be reproduced from the respective record tracks, the light spot is formed having an extremely small spot diameter, the record tracks can be arranged so as to mutually close together and with high density. Thus, the more higher density optical information can be recorded or reproduced.
A U.S. Pat. No. 3,851,951 issued on Dec. 3, 1974 exemplifies means for producing the light spot to scan the record tracks in the direction traverse to the running direction of the optical record medium using an acoustooptic deflection element of an anisotorphic Bragg diffraction type.
The acoustooptic deflection element generally has a rectangular shape in a longitudinal cross section and a drive signal S.sub.1 is supplied to a transducer installed on one end surface thereof, the drive signal S.sub.1 being formed of saw-tooth wave such that a frequency f is changed from a lower limit frequency f.sub.1 to an upper limit frequency f.sub.2 with time (refer to FIG. 8). A sonic pattern 3B whose density is gradually changed in the elongated direction c of the element 3 is formed in the acoustooptic deflection element 3 and is travelled in a direction c toward which an absorbing member 3C is installed on the other end surface of the deflection element 3 (refer to FIG. 7).
The sonic pattern 3B acts as a diffraction grating when each light beam LB1 incident on an incident surface 3D constituted by a linear polarization on the acoustooptic deflection element 3 transmits into the acoustooptic deflection element 3. Therefore, the light beams are diffracted by means of the sonic pattern 3B, so that a diffraction angle of a first-order diffraction light LB3 with respect to a zero-order diffraction light LB2 is changed as the density of the sonic pattern 3B is changed according to the change in the frequency f of the drive signal S1. Consequently, the light spot 2 can be achieved which scans along the elongated direction d of the acoustooptic deflection element 3. However, the previously proposed optical information processing systems have problems described below.
When the optical information is reproduced from the record tracks, a so-called tracking error occurs in the light spot. The tracking error causes a bit error during the reproduction of the optical information and in a worst case the reproduction of the optical information becomes difficult.
To solve the above-described tracking error problem, an amount of deviation of a scanning trajectory of the light spot from a center line of each record track (that is to say, a tracking error) is detected and the light spot is controlled in such a way that the scanning trajectory of the light spot has aligned with the center line of each record track TR (hereinafter referred also to as a just tracking state).
For example, a technique of tracking control used in, e.g., a video tape recorder is applied to the optical information processing system in such a way that a phase control for a running system of the optical record medium is carried out for the tracking control of the record tracks.
However, in a case where the phase of the running system is controlled, it is difficult to increase a speed of response in the running system. In addition, the phase control cannot actually be applied to such kinds of the optical information processing systems.
Furthermore, it is necessary to perform the tracking control with high gain and with high accuracy in such kinds of the optical information processing systems.
In this case, if a gain of the tracking control is increased and a range of frequency of a response in a tracking control system is extended to achieve a high accuracy of the tracking control, it is difficult to increase the response speed of the running system. Therefore, a jitter occurs during the run of the optical recording medium.
On the other hand, a method has been proposed in which an object lens is arranged between the acoustooptic deflection element and optical record medium and the object lens is driven in response to a detection result of the tracking error.
However, when the above-described method is applied to such optical information processing systems, a drive system to drive the object lens becomes necessary and the whole construction accordingly becomes complex.
In addition, such a problem as to increase the response speed of the tracking control actually occurs when the object lens is driven.
Another method has been proposed in which a galvano mirror is used in place of the acoustooptic deflection element to produce a scanning light beam and the galvano mirror is driven and controlled on the basis of the tracking error.
However, in the above-described other method, a scale of the optical system having the galvano mirror is enlarged.
The whole construction of the optical information processing system becomes complex.
When the light beams are deflected by means of the galvano mirror a light path difference occurs in the scanning light beams and a drive system such as an actuator becomes necessary.
Consequently, the whole construction accordingly becomes enlarged.